Antenna Port Mapping for Demodulation Reference Signals

ABSTRACT

A unified, rank independent mapping between antenna ports and group/code pairs. Each antenna port is uniquely associated with one code division multiplexing (CDM) group and one orthogonal cover code (OCC). The mapping between antenna ports and group/code pairs is chosen such that, for a given antenna port, the CDM group and OCC will be the same for every transmission rank.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/280,216, filed Feb. 20, 2019, which is a continuation of U.S. patentapplication Ser. No. 15/088,603, filed Apr. 1, 2016, now U.S. Pat. No.10,244,424, which is a continuation of U.S. patent application Ser. No.13/874,768, filed May 1, 2013, now U.S. Pat. No. 9,307,542, which is acontinuation of U.S. patent application Ser. No. 12/900,628, filed Oct.8, 2010, now U.S. Pat. No. 8,446,886, which claims benefit ofPCT/CN2010/000084, filed Jan. 20, 2010, the disclosures of all of whichare incorporated by reference herein in their entirety.

BACKGROUND

The present invention relates generally to demodulation referencesignals (DM-RSs) for LTE and LTE advanced communication systems and,more particularly, to the configuration of antenna ports foruser-specific DM-RSs.

The 3rd Generation Partnership Project (3GPP) is responsible for thestandardization of UMTS (Universal Mobile Telecommunication Service)system and LTE (Long Term Evolution). LTE is a communication technologyfor realizing high-speed packet-based communication that can reach highdata rates both in the downlink and in the uplink, which is thought as anext generation mobile communication system of the UMTS system. The 3GPPwork on LTE is also referred to as E-UTRAN (Evolved UniversalTerrestrial Access Network). The first release of LTE, referred to asrelease-8 (Rel-8) can provide peak rates of 100 Mbps, a radio-networkdelay of, e.g., 5 ms or less, a significant increase in spectrumefficiency and a network architecture designed to simplify networkoperation, reduce cost, etc. In order to support high data rates, LTEallows for a system bandwidth of up to 20 MHz. LTE is also able tooperate in different frequency bands and can operate in both FDD(Frequency Division Duplex) and TDD (Time Division Duplex) modes. Themodulation technique or the transmission scheme used in LTE is known asOFDM (Orthogonal Frequency Division Multiplexing).

For the next generation mobile communications system, e.g., IMT-advanced(International Mobile Telecommunications) and/or LTE-advanced, which isan evolution of LTE, support for bandwidths of up to 100 MHz is beingdiscussed. LTE-advanced can be viewed as a future release of the LTEstandard and since it is an evolution of LTE, backward compatibility isimportant so that LTE-advanced can be deployed in spectrum alreadyoccupied by LTE. In both LTE and LTE-advanced radio base stations knownas evolved NodeBs (eNBs or eNodeBs), multiple-input, multiple output(MIMO) antenna configurations and spatial multiplexing can be used inorder to provide high data rates to user terminals. Another example of aMIMO-based system is WiMAX (Worldwide Interoperability for MicrowaveAccess) system.

To carry out coherent demodulation of different downlink physicalchannels, the user terminal needs estimates of the downlink channel.More specifically, in the case of OFDM transmissions, the user terminalneeds an estimate of the complex channel of each subcarrier. One way toenable channel estimation in the case of OFDM transmissions is to insertknown reference symbols into the OFDM time frequency grid. In LTE, thesereference symbols are jointly referred to as downlink reference signals.

Two types of downlink reference signals are used in LTE systems: cellspecific downlink reference signals and user specific downlink referencesignals. Cell specific downlink reference signals are transmitted inevery downlink subframe, and span the entire downlink cell bandwidth.The cell specific reference signals can be used for channel estimationand coherent demodulation except when spatial multiplexing is used. Auser terminal specific reference signal is used for channel estimationand demodulation of the downlink shared channel when spatialmultiplexing is used. The user specific reference signals aretransmitted within the resource blocks assigned to the specific userterminal for transmitting data on the downlink shared channel. The userterminal specific reference signals are subject to the same precoding asdata signals transmitted to the user terminal. The present invention isapplicable to user terminal specific downlink reference signals.

FIG. 1 illustrates a portion of an exemplary OFDM time-frequency grid 50for LTE. Generally speaking, the time-frequency grid 50 is divided intoone millisecond subframes. One subframe is shown in FIG. 1. Eachsubframe includes a number of OFDM symbols. For a normal cyclic prefix(CP) link, suitable for use in situations where multipath dispersion isnot expected to be extremely severe, a subframe comprises fourteen OFDMsymbols. A subframe comprises twelve OFDM symbols if an extended cyclicprefix is used. In the frequency domain, the physical resources aredivided into adjacent subcarriers with a spacing of 15 kHz. The numberof subcarriers varies according to the allocated system bandwidth. Thesmallest element of the time-frequency grid 50 is a resource element. Aresource element comprises one OFDM symbol on one subcarrier.

For purposes of scheduling transmission on the downlink shared channel(DL-SCH), the downlink time-frequency resources are allocated in unitscalled resource blocks (RBs). Each resource block spans twelvesubcarriers (which may be adjacent or distributed across the frequencyspectrum) and one-half of one subframe. The term “resource block pair”refers to two consecutive resource blocks occupying an entire onemillisecond subframe.

Certain resource elements within each subframe are reserved for thetransmission of downlink reference signals. FIG. 1 illustrates oneexemplary resource allocation pattern for the downlink reference signalsto support downlink transmissions up to rank 4. Twenty-four resourceelements within a subframe are reserved for transmission of the downlinkreference signals. More specifically, the demodulation reference signalsare carried in OFDM symbols 5, 6, 12, and 13 (i.e., the sixth, seventh,thirteenth, and fourteenth symbols) of the OFDM subframe. The resourceelements for the demodulation reference signals are distributed in thefrequency domain.

The resource elements for the demodulation reference signals are dividedinto two code division multiplexing (CDM) groups referred to herein asCDM Group 1 and CDM Group 2. In LTE systems supporting transmissionranks from 1-4, two CDM groups are used in combination with length-2orthogonal cover codes (OCCs). The orthogonal cover codes are applied toclusters of two reference symbols. The term “cluster” as used hereinrefers to groupings of adjacent (in the time domain) reference symbolsin the same subcarrier. In the embodiment shown in FIG. 1, thesubcarriers containing demodulation reference symbols include twoclusters each.

FIG. 2 illustrates an exemplary allocation of resource elements for aspatial multiplexing system supporting transmission ranks up to eight.It may be noted that the resource allocation pattern is the same as theallocation pattern shown in FIG. 1. To support higher transmissionranks, a length-4 OCC is used instead of a length-2 OCC. The length-4OCC is applied across two clusters of resource elements.

Up to eight antenna ports may be defined to support up to 8 spatiallayers. The 8 antenna ports can be mapped to two CDM groups, each usingfour OCCs. Thus, the antenna ports can be uniquely identified by twoparameters, i.e., CDM group index and OCC index, referred to herein asan index pair. Currently, the mapping between antenna ports and indexpairs has not been specified in the LTE standard. Some mappings may berank dependent, which requires that different port mappings be used foreach transmission rank. Using different port mappings for differenttransmission ranks imposes a burden on the user terminal, which mustperform channel estimation differently when the transmission rankschanges.

BRIEF SUMMARY

The present invention provides a unified, rank independent mappingbetween antenna ports and group/code pairs. Each antenna port isuniquely associated with one code division multiplexing (CDM) group andone orthogonal cover code (OCC). The mapping between antenna ports andgroup/code pairs is chosen such that, for a given antenna port, the CDMgroup and OCC will be the same for every transmission rank.

One exemplary embodiment of the invention comprises a method implementedby a base station for transmitting demodulation reference signals to auser terminal. The method comprise determining a transmission rank for adownlink transmission to said user terminal; determining one or morereference signal antenna ports for said downlink transmission based onsaid transmission rank, wherein each port is defined by an group/codepair comprising a code division multiplexing group and orthogonal covercode; mapping reference signal antenna ports to group/code pairs foreach transmission rank such that the code division multiplexing groupand code orthogonal cover code are the same for a given antenna port forevery transmission rank; and transmitting said downlink referencesymbols over said reference signal antenna ports.

Yet another exemplary embodiment of the invention comprises a basestation configured to implement the method described above.

Another exemplary embodiment of the invention comprises a methodimplemented by a user terminal for receiving demodulation referencesignals transmitted by a base station. The user terminal methodcomprises determining a transmission rank for a downlink transmission tosaid user terminal; determining one or more reference signal antennaports for said downlink transmission based on said transmission rank,wherein each port is defined by an group/code pair comprising a codedivision multiplexing group and orthogonal cover code; mapping referencesignal antenna ports to group/code pairs for each transmission rank suchthat the code division multiplexing group and orthogonal cover code arethe same for a given antenna port for every transmission rank; andreceiving said downlink reference symbols over said reference signalantenna ports corresponding to the transmission rank.

Yet another exemplary embodiment of the invention comprises a userterminal configured to implement the method described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the allocation of resource elements in an OFDM systemfor demodulation reference signals to support transmission ranks up to4.

FIG. 2 illustrates the allocation of resource elements in an OFDM systemfor demodulation reference signals to support transmission ranks up to8.

FIG. 3 illustrates an exemplary MIMO communication system.

FIG. 4 illustrates an exemplary transmit signal processor for an OFDMsystem.

FIGS. 5a-5b illustrates the mapping of codewords to layers according toone exemplary embodiment for transmission ranks from 1 to 8.

FIGS. 6a-6b illustrates the mapping of codewords to layers according toone exemplary embodiment for transmission ranks from 1 to 8.

FIG. 7 illustrates an exemplary method for transmitting demodulationreference signals.

FIG. 8 illustrates a method of receiving demodulation reference signals

DETAILED DESCRIPTION

FIG. 3 illustrates a multiple input/multiple output (MIMO) wirelesscommunication system 10 including a base station 12 (called an evolvedNodeB in LTE), and a user terminal 14. The present invention will bedescribed in the context of an LTE system, although the presentinvention is applicable to other types of communication systems. Thebase station 12 includes a transmitter 100 for transmitting signals tothe second station 14 over a communication channel 16, while the userterminal 14 includes a receiver 200 for receiving signals transmitted bythe base station 12. Those skilled in the art will appreciate that thebase station 12 and user terminal 14 may each include both a transmitter100 and receiver 200 for bi-directional communications.

An information signal is input to the transmitter 100 at the basestation 12. The transmitter 100 includes a controller 110 to control theoverall operation of the transmitter 100 and a transmit signal processor120. The transmit signal processor 120 performs error coding, maps theinput bits to complex modulation symbols, and generates transmit signalsfor each transmit antenna 130. After upward frequency conversion,filtering, and amplification, transmitter 100 transmits the transmitsignals from respective transmit antennas 130 through the communicationchannel 16 to the user terminal 14.

The receiver 200 at the user terminal 14 demodulates and decodes thesignals received at each antenna 230. Receiver 200 includes a controller210 to control operation of the receiver 200 and a receive signalprocessor 220. The receive signal processor 220 demodulates and decodesthe signal transmitted from the first station 12. The output signal fromthe receiver 200 comprises an estimate of the original informationsignal. In the absence of errors, the estimate will be the same as theoriginal information signal input at the transmitter 12.

In LTE systems, spatial multiplexing can be used when multiple antennasare present at both the base station 12 and the user terminal 14. FIG. 4illustrates the main functional components of a transmit signalprocessor 120 for spatially multiplexing. The transmit signal processor120 comprises a layer mapping unit 122, a precoder 124, and resourcemapping units 128. A sequence of information symbols (data symbols orreference symbols) is input to the layer mapping unit 122. The symbolsequence is divided into one or two codewords. The layer mapping unit122 maps the codewords into N_(L) layers depending on the transmissionrank. It should be noted that the number of layers does not necessarilyequal the number of antennas 130. Different codewords are typicallymapped to different layers; however, a single codeword may be mapped toone or more layers. The number of layers corresponds to the selectedtransmission rank. After layer mapping, a set N_(L) symbols (one symbolfrom each layer) are linearly combined and mapped to N_(A) antenna ports126 by precoder 124. The combining/mapping is described by a precodermatrix of size N_(A)×N_(L). The resource mapping unit 128 maps symbolsto be transmitted on each antenna port 126 to the resource elementsassigned by the MAC scheduler.

When a user terminal 14 is scheduled to receive a downlink transmissionon the downlink shared channel (DL-SCH), the MAC scheduler at thetransmitting station 12 allocates one or more resource block pairs tothe user terminal 14. As previously noted, certain resources elements ineach resource block are reserved for downlink reference signals. Tosupport downlink transmission containing up to eight layers, userterminal specific downlink reference signals are needed for eightlayers. According to the present invention, eight distinct referencesignal antenna ports are defined to support transmissions with up toeight layers. Each antenna port is uniquely associated with one codedivision multiplexing (CDM) group and one orthogonal cover code (OCC).The OCC may comprise, for example, a length-2 or length-4 Walsh code,although other orthogonal codes could also be used. For convenience, theCDM groups may be identified by a group index having a value from 1 to2, and the OCC may be identified by a code index having a value from 1to 4. The combination of a CDM group and OCC is referred herein as agroup/code pair.

In the exemplary embodiment, there are two CDM groups and 4 OCCs. Thus,there are eight possible combinations of CDM groups and OCCs (2 groups×4OCCs) so that eight layers can be supported. The mapping between antennaports and group/code pairs is designed to be rank independent. Morespecifically, the mapping between antenna ports and group/code pairs ischosen such that, for a given antenna port, the CDM group and OCC willbe the same for every transmission rank.

Table 1 below and FIGS. 5a and 5b illustrate one possible mappingbetween antenna ports and group/code pairs according to one embodimentof the present invention.

TABLE 1 Antenna Port Mapping Antenna port CDM Group OCC 1 1 1 2 1 2 3 21 4 2 2 5 1 3 6 1 4 7 2 3 8 2 4

The OCCs are the Walsh codes given by the Walsh code matrix:

$\begin{bmatrix}1 & 1 & 1 & 1 \\1 & {- 1} & 1 & {- 1} \\1 & {- 1} & {- 1} & 1 \\1 & 1 & {- 1} & {- 1}\end{bmatrix}\begin{matrix}{{OCC}\; 1} \\{{OCC}\; 2} \\{{OCC}\; 3} \\{{OCC}\; 4}\end{matrix}$

The antenna port mapping shown in Table 1 allocates CDM group 1 to ports1, 2, 5, and 6 and CDM group 2 to ports 3, 4, 7, and 8. OCC 1 isallocated to ports 1 and 3, OCC2 is allocated to ports 2 and 4, OCC 3 isallocated to ports 5 and 7, and OCC 4 is allocated to ports 6 and 8.

This antenna port mapping described above is rank independent so that agiven antenna port will always use the same CDM group and OCC regardlessof the transmission rank. Further, the antenna ports associated with aparticular CDM group possess a nesting property. That is, for the set ofthe antenna ports associated with a given CDM group, the antenna portsused for a low transmission rank will be a subset of the antenna portsused for a higher transmission rank. Thus, for the antenna portsassociated with CDM group 1, the ports used for transmission rank 1 area subset of the ports used for transmission rank 2, which are a subsetof the ports used for transmission rank 5, which are a subset of theports used for transmission rank 6. The same nesting property applies tothe antenna ports associated with CDM group 2.

Another useful property of the antenna port mapping shown above is thatthe length-4 OCCs on certain antenna ports are identical to length-2OCCs. For example, for transmission rank 2, the length-4 Walsh codes onantenna ports 1 and 2 appear the same as length-2 Walsh codes. In thecase of single-user MIMO systems, this property enables the userterminal 14 to use length-2 OCCs to perform channel estimation. Usinglength-2 OCCs for channel estimation allows the receiver 200 tointerpolate and thus provide more accurate channel estimates. Improvedchannel estimation is beneficial for high mobility user terminals 14.Thus, for transmission ranks 2, 4 and 5, the receiver may use length-2Walsh codes to perform channel estimation on antenna ports 1 and 2 asshown in FIG. 5. Similarly, for transmission ranks 3 and 4, the receivermay use length-2 Walsh codes to perform channel estimation on antennaports 3 and 4. When more than two layers are multiplexed into one CDMgroup, length-4 OCC should be used for channel estimation.

Table 2 below and FIGS. 6a and 6b illustrate an alternate antenna portmapping according to another exemplary embodiment of the invention.

TABLE 2 Antenna Port Mapping Antenna port CDM Group OCC 7 1 1 8 1 2 9 21 10 2 2 11 1 3 13 1 4 12 2 3 14 2 4

In this alternative antenna port mapping, the OCCs are the Walsh codesgiven by the Walsh code matrix:

$\begin{bmatrix}1 & 1 & 1 & 1 \\1 & {- 1} & 1 & {- 1} \\1 & 1 & {- 1} & {- 1} \\1 & {- 1} & {- 1} & 1\end{bmatrix}\begin{matrix}{{OCC}\; 1} \\{{OCC}\; 2} \\{{OCC}\; 3} \\{{OCC}\; 4}\end{matrix}$

The antenna port mapping shown in Table 2 allocates CDM group 1 to ports7, 8, 11, and 13 and CDM group 2 to ports 9, 10, 12, and 14. OCC 1 isallocated to ports 7 and 9, OCC2 is allocated to ports 8 and 10, OCC 3is allocated to ports 11 and 12, and OCC 4 is allocated to ports 13 and14.

It should be noted that CDM/OCC allocation only are considered herewithout regard to OCC mapping. With OCC mapping, OCC allocation could bevarying from subcarrier to subcarrier in the frequency domain.

For multi-user MIMO, the user terminal 14 may not know whether otheruser terminals 14 are co-scheduled at the same time, such as whentransparent MU-MIMO is used. This lack of knowledge forces each userterminal 14 to use length-4 OCC for channel estimation even for lowerrank, which can degrade performance a bit more, especially for highspeed case. In order to exploit the advantage of length-2 OCC, wepropose to introduce 1-bit OCC length flag in control signaling toprovide the user terminal 14 some more information on OCC details, whichcan accordingly improve the performance in MU-MIMO. Therefore, this1-bit flag can also enable dynamic SU/MU switching well.

FIG. 7 illustrates an exemplary method 150 implemented by the basestation 12 for transmitting demodulation reference signals to a userterminal 14. When a user terminal 14 is scheduled to receive a downlinktransmission on the downlink shared channel (DL-SCH), the base station12 determines the transmission rank for the downlink transmission to theuser terminal 14 (block 152) and determines one or more reference signalantenna ports for the downlink transmission based on the transmissionrank (block 154). The transmit signal processor 130 at the base station12 is configured to map the antenna ports to a particular CDM group andorthogonal cover code such that the CDM group and orthogonal cover codeare the same for a given antenna port for every transmission rank. Thetransmit signal processor 130 maps the demodulation reference signal tothe reference signal antenna ports (block 156) corresponding to thetransmission rank and transmits the demodulation reference signals overthe selected antenna ports (block 158).

FIG. 8 illustrates an exemplary procedure 160 implemented by a userterminal 14 to receive downlink reference signals from the base station12. The user terminal 14 determines the transmission rank for thedownlink transmission to the user terminal (block 162) and selects oneor more reference signal antenna ports based on the transmission rank(block 164). The receive signal processor 230 is configured to map thereference signal antenna ports to a CDM group and OCC such that the CDMgroup and OCC are the same for a given antenna port for everytransmission rank (block 166). The receive signal processor 230 receivesthe reference signals over the selected antenna ports (block 168) andprocess the signals.

The antenna port mapping is applicable to both single-user MIMO andmulti-user MIMO. It is also applicable to DwPTS and extended CPs, aswell as multiple component carriers. The antenna port mapping scheme canbe used to reduce the peak power randomization effect.

The present invention may, of course, be carried out in other specificways than those herein set forth without departing from the scope andessential characteristics of the invention. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive, and all changes coming within the meaning and equivalencyrange of the appended claims are intended to be embraced therein.

What is claimed is:
 1. A method, implemented by a base station, fortransmitting demodulation reference signals to a user terminal, themethod comprising: determining a selected transmission rank from aplurality of transmission ranks for a downlink transmission to the userterminal; determining one or more antenna ports for the downlinktransmission, wherein each antenna port is defined by a group and codepair in a set of group and code pairs, wherein each group and code pairin the set of group and code pairs comprises a code divisionmultiplexing group and orthogonal cover code; for each transmissionrank, mapping the antenna ports to the group and code pairs such themapping of the antenna ports to the group and code pairs for a highestone of the transmission ranks is a superset of the mappings of theantenna ports to the group and code pairs for all lower transmissionranks; and transmitting the demodulation reference signals over the oneor more antenna ports corresponding to the selected transmission rank.2. The method of claim 1, wherein the mapping of the antenna ports tothe group and code pairs for each of the lower transmission ranks is asubset of mapping of the antenna ports to the group and code pairs forthe next highest transmission rank.
 3. The method of claim 1: whereinthe orthogonal cover codes comprise length-4 cover codes; and whereinthe mapping of the one or more antenna ports to the one or morecorresponding group and code pairs is such that, for selected antennaports, the length-4 orthogonal cover codes comprise two length-2 covercodes for channel estimation.
 4. The method of claim 2, furthercomprising sending a control signal to the user terminal to indicatewhether channel estimation is performed using length-2 or length-4orthogonal cover codes for the determined one or more antenna ports. 5.A method, implemented by a user terminal, for receiving demodulationreference signals transmitted by a base station, the method comprising:determining a selected transmission rank from a plurality oftransmission ranks for a downlink transmission to the user terminal;determining one or more antenna ports for the downlink transmission,wherein each antenna port is defined by a group and code pair in a setof group and code pairs, wherein each group and code pair in the set ofgroup and code pairs comprises a code division multiplexing group andorthogonal cover code; for each transmission rank, mapping the antennaports to the group and code pairs such the mapping of the antenna portsto the group and code pairs for a highest one of the transmission ranksis a superset of the mappings of the antenna ports to the group and codepairs for all lower transmission ranks; and receiving the demodulationreference signals over the one or more antenna ports corresponding tothe selected transmission rank.
 6. The method of claim 5, the mapping ofthe antenna ports to the group and code pairs for each of the lowertransmission ranks is a subset of mapping of the antenna ports to thegroup and code pairs for the next highest transmission rank.
 7. Themethod of claim 5: wherein the orthogonal cover codes comprise length-4cover codes; and wherein the mapping of the one or more antenna ports tothe one or more corresponding group and code pairs is such that, forselected antenna ports, the length-4 orthogonal cover codes comprise twolength-2 cover codes for channel estimation.
 8. The method of claim 7,further receiving a control signal from the base station and performingchannel estimation using either length-2 or length-4 orthogonal covercodes for the determined one or more antenna ports depending on thecontrol signal.
 9. A base station, comprising: a plurality of transmitantennas; a transmitter operatively connected to the transmit antennas,the transmitter including a transmit signal processor and transmitcontroller; the transmitter configured to: determine a selectedtransmission rank from a plurality of transmission ranks for a downlinktransmission to a user terminal; determine one or more antenna ports forthe downlink transmission, wherein each antenna port is defined by agroup and code pair in a set of group and code pairs, wherein each groupand code pair in the set of group and code pairs comprises a codedivision multiplexing group and orthogonal cover code; for eachtransmission rank, map the antenna ports to the group and code pairssuch the mapping of the antenna ports to the group and code pairs for ahighest one of the transmission ranks is a superset of the mappings ofthe antenna ports to the group and code pairs for all lower transmissionranks; and transmit demodulation reference signals over the one or moreantenna ports corresponding to the selected transmission rank.
 10. Thebase station of claim 9, wherein the mapping of the antenna ports to thegroup and code pairs for each of the lower transmission ranks is asubset of mapping of the antenna ports to the group and code pairs forthe next highest transmission rank.
 11. The base station of claim 9:wherein the orthogonal cover codes comprise length-4 cover codes; andwherein the mapping of the one or more antenna ports to the one or morecorresponding group and code pairs is such that, for selected antennaports, the length-4 orthogonal cover codes comprise two length-2 covercodes for channel estimation.
 12. The base station of claim 11, whereinthe base station is further configured to send a control signal to auser terminal to indicate whether channel estimation is performed usinglength-2 or length-4 orthogonal cover codes for the determined one ormore antenna ports.
 13. A user terminal, comprising: a plurality ofreceive antennas; a receiver operatively connected to the receiveantennas, the receiver including a receive signal processor and receivecontroller; wherein the receiver is configured to: determine a selectedtransmission rank from a plurality of transmission ranks for a downlinktransmission to the user terminal; determine one or more antenna portsfor the downlink transmission, wherein each antenna port is defined by agroup and code pair in a set of group and code pairs, wherein each groupand code pair in the set of group and code pairs comprises a codedivision multiplexing group and orthogonal cover code; for eachtransmission rank, map the antenna ports to the group and code pairssuch the mapping of the antenna ports to the group and code pairs for ahighest one of the transmission ranks is a superset of the mappings ofthe antenna ports to the group and code pairs for all lower transmissionranks; and receive demodulation reference signals over the one or moreantenna ports corresponding to the selected transmission rank.
 14. Theuser terminal of claim 13 wherein the mapping of the antenna ports tothe group and code pairs for each of the lower transmission ranks is asubset of mapping of the antenna ports to the group and code pairs forthe next highest transmission rank.
 15. The user terminal of claim 13:wherein the orthogonal cover codes comprise length-4 cover codes; andwherein the mapping of the one or more antenna ports to the one or morecorresponding group and code pairs is such that, for selected antennaports, the length-4 orthogonal cover codes comprise two length-2 covercodes for channel estimation.
 16. The user terminal of claim 15, whereinthe receiver is further configured to receive a control signal from thebase station and perform channel estimation using either length-2 orlength-4 orthogonal cover codes for the determined one or more antennaports depending on the control signal.